Modular shoulder prosthesis

ABSTRACT

A modular shoulder prosthesis including a head having a semi-spherical articulation surface bounded by an articular margin disposed in an articular plane generally normal to a head axis. The head further includes a backside disposed opposite the articulation surface and separated from the articulation surface by the articular margin. The prosthesis also includes a stem portion with a proximal end and a distal shaft for insertion into a medullary canal along a shaft axis. A coupling structure is adapted to removably attach the head to the stem through motion in the articular plane.

This invention relates generally to bone prostheses and moreparticularly to a modular shoulder prosthesis.

BACKGROUND OF THE INVENTION

When a joint, such as the hip or shoulder, becomes impaired due toarthritis, disease or trauma, it is sometimes necessary to replace allor part of the joint with a prosthesis to restore function. Forinstance, hip replacement, where a prosthesis is provided to replace thefemoral head and in some cases all or part of the acetabulum, has becomea common procedure to treat femoral head fractures and arthritis inelderly patients. As a result of anatomical constraints and challengesin the shoulder, shoulder implants have historically been much lesssuccessful and less common than hip replacements. Recently, however,shoulder arthroplasty has emerged as an accepted treatment for severearthritis and humeral head fractures.

As a consequence of the increasing acceptance of shoulder prostheses,many different devices have been developed to address various problemsthat have arisen and to offer additional benefits and features. In thesimplest form, a shoulder prosthesis is formed as a single piece with ahead to articulate with the glenoid cavity, and a stem to extend downthe medullary canal of the humerous and support the head. While simpleto construct, unitary implants do not offer any adjustability toaccommodate the natural variations in size and geometry that occur amongjoints of different patients. To accommodate these variations, a largestock of devices must be manufactured and maintained to insure that anadequate match can be achieved during an operation. Stocking the largenumber of devices is a significant expense with one-piece designs, andin some cases a surgeon may not be provided with sufficient flexibilityto achieve an ideal fit to the patient.

To avoid the expense of maintaining a large stock of single-pieceprosthetics and to provide increased flexibility to surgeons, manyshoulder implant makers have gone to a modular design that is assembledduring the operation from two or three pieces. These pieces include ahead to articulate with the glenoid and a stem structure on which thehead is mounted and secured to the bone. In some cases, the stemincludes a separate body portion disposed between the head and anintermedullary portion of the stem that extends down the medullarycanal. By utilizing a modular design, a wide variety of devices can beassembled from only a few pieces, thus providing increased flexibilityto accommodate anatomical variation and eliminating much of the costassociated with maintaining a large selection of one-piece devices.

One drawback of existing modular implants is the difficulty of reliablyand easily attaching the pieces together. With existing designs, thepieces are most commonly held together with a taper-lock structure. Inparticular, the backside of the head is provided with a male or femaletaper, and a mating structure is provided on the top of the stem. Afterselecting the appropriate components, the surgeon places the head on thestem and drives the pieces together to lock them in place.Unfortunately, because the components are held together only byfriction, it is possible for them to become loosened or dislocated afterinstallation, in which case another operation must be performed torestore the implant.

Another drawback with taper-lock modular designs is that it can bedifficult to disassemble an implant and install a new head withoutremoving the stem from the bone. In particular, it is often difficult toimpart enough force to the head to separate the taper-lock withoutdislodging the stem from the bone at the same time. Moreover, to beremoved, the head must be lifted away from the stem by the length of thetaper and, thus, the joint must be dislocated to permit the necessaryseparation. As a result, the recovery time is greatly extended over whatwould be required if dislocation were not necessary.

Another common feature of many existing taper-lock designs is a proximalflange attached to the top of the stem adjacent the head. This flangeprevents the implant from subsiding down into the femur and avoids theresulting upward force on the bottom of the head which would tend toseparate the taper lock. Unfortunately, over time, bony ingrowth canoccur around the underside of the flange and the sides of the stem.Although this bony ingrowth is beneficial in that it helps to stabilizethe implant, it also makes the implant much more difficult to removewhen a revision is necessary. In particular, the flange blocks thesurgeon from slipping a chisel down the bone adjacent the sides of theimplant to separate the implant from the bone. As a result, asignificant amount of bone may be dislodged with the implant, making itmore difficult to secure the replacement implant.

The flanges used on many implants are also problematic because theydecrease the thickness available for the head. The natural humeral headis typically 16-18 mm thick. Typical flanges are around 3 mm thick and,with a taper-lock device, a gap of approximately 2 mm must be leftbetween the bottom of the head and the collar to accommodate themachining tolerances in the taper. As a result, the implant head isoften significantly thinner than the original anatomy. This can limitjoint mobility and increase the chance of dislocation.

In addition to the specific drawbacks associated with various existingimplant designs, there are a number of general problems inherent inshoulder replacements. In particular, it is generally difficult toestablish the proper position and orientation for the implant in thehumerus. One of the more important variables is the rotational position,or retroversion, of the head on the humerus. Anatomically, the averageretroversion between a plane defined by the perimeter of the anatomicalhead and the axis of the flexed forearm is approximately 30-degrees.Unfortunately, with existing implants and techniques for theirinstallation, it has been very difficult to reliably reproduce desiredretroversion. Establishing correct retroversion is important becauseincorrect retroversion can lead to problems with subsequent dislocation.

In addition to the retroversion of the implant, it is necessary toestablish the correct height of the implant on the humeral shaft. Withexisting designs, the surgeon slips the stem into the medullary canaland makes an educated guess at the proper height. Excess height maycreate too much tension in the deltoid, while inserting the implant toofar down the humerus can result in deltoid lag. Similarly, the offset ofthe face of the head relative to the stem must be established correctlyor excess or insufficient tension in the rotator cuff may be created.Unfortunately, with existing designs there is no way to evaluate implantheight or head offset prior to final installation, after whichcorrection is difficult.

SUMMARY OF THE INVENTION

The present invention is a modular shoulder prosthesis including a headhaving a semi-spherical articulation surface bounded by an articularmargin disposed in an articular plane generally normal to a head axis.The head further includes a backside disposed opposite the articulationsurface and separated from the articulation surface by the articularmargin. The prosthesis also includes a stem portion with a proximal endand a distal shaft for insertion into a medullary canal along a shaftaxis. A coupling structure is adapted to removably attach the head tothe stem through motion in the articular plane.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is an exploded isometric view of a modular shoulder implantconstructed according to the present invention.

FIG. 2 shows a modular shoulder implant kit constructed according to thepresent invention.

FIG. 3 is a cross-sectional view of a shaft of the implant of FIG. 1.

FIGS. 4-9 are various views of a body portion of the implant of FIG. 1.

FIGS. 10-13 are various views of a head portion of the implant of FIG.1.

FIG. 14 is an isometric view of the backside of the head of FIGS. 10-13.

FIG. 15 is an isometric view of the head partially installed on thebody.

FIG. 16 is a cross-sectional view of the implant along line 8--8 of FIG.3.

FIG. 17 is a medial elevational view of the implant of FIG. 1.

FIG. 18 is an isometric view of a targeting/installation instrumentaccording to the present invention.

FIG. 19 is a lateral elevational view of the targeting/installationinstrument of FIG. 10.

FIG. 20 is an elevational view from above of the targeting installationinstrument of FIG. 18.

FIG. 21 is an isometric view of the implant of FIG. 1 in an assembledconfiguration.

DETAILED DESCRIPTION

A shoulder implant constructed according to the present invention isshown generally at 10 in FIG. 1. Implant 10 includes a head 12 and astem 14. The stem preferably includes a distal shaft 16 and a body 18.The components making up implant 10 are preferably chosen from a kit 20of interchangeable shafts, bodies and heads, as shown in FIG. 2. Byselecting an appropriate shaft, body and head from kit 20, a surgeon isable to create an implant that is sized properly for almost any patient.It should be noted that positional references such asanterior/posterior, medial/lateral and proximal/distal used herein aremade with reference to an implant as it would be positioned in apatient.

Shaft 16 is shown in greater detail in FIG. 3 and includes a proximaltapered end 30 extending distally to a shoulder 32 which tapers smoothlyinto a cylindrical medial region 34 with distal locking holes 36, 38. Ascan be seen in FIG. 2, the shaft can have a medial region of varyingdiameter and/or varying length. Generally speaking, the longer shaftsare used where there is a mid-shaft fracture in addition to the proximaltrauma. The varying diameter short shafts are used to accommodate sizevariations in the proximal end of the humerus. Either or both of holes36, 38 may be elongated to allow for movement of the medial region overthe locking screws. This is normally desirable when the implant is usedto treat a combined mid-shaft fracture.

A rounded and tapered distal tip 40 is formed on the end of medialregion 34. Shaft 16 preferably includes a central canulation 42 whichcan be used to guide the implant into the humerus with the aid of aguide wire. As best shown in FIG. 1, an alignment notch 44 is formed inshoulder 32 to aid in establishing the correct orientation of the bodyon the shaft, as will be described below. A threaded hole 46 is formedin tapered end 30 to receive a screw 50 which is used to draw the bodyfirmly onto the tapered end. A wiring hole 48 is provided just distal ofshoulder 32 to allow tension band wiring to be secured through theimplant. In addition, when the implant is to be cemented in place, aK-wire can be driven through humerus and hole 48 to fix the position ofthe implant while the cement cures.

As indicated in FIG. 1, body 18 mounts to the top of shaft 16. Referringto FIGS. 4-9, body 18 has a distal end 54 with a cylindrical taperingsocket 56 extending upwardly therefrom into the body. Socket 56 is sizedto receive tapered end 30 of shaft 16 and taper-lock thereto to allowthe body to be securely mounted to the shaft. A proximal bore 58 extendsfrom the socket to the top of the body to the previously discussed screwto engage the top of the shaft to draw it into the socket. A small rib60 is provided in the bore to engage against the head of the screw.

A small finger 62 projects down from the distal end of body 18 adjacentthe socket to engage alignment notch 44 as the body is installed on theshaft. See FIGS. 1 and 17. This ensures the proper rotationalpositioning of the body on the shaft so that the various holes in theshaft are oriented correctly. Body 18 further includes a lateral rib 64with three suture holes 66 which aid in securing the fracture fragmentsto the implant. Upper and lower medial suture holes 68, 70 are alsoprovided in body 18 to offer additional options in securing thefragments. A medially-positioned, anteriorly-oriented threaded hole 72is formed in body 18 to receive a screw for securing the head to thebody. Hole 72 also serves as a mounting point for atargeting/installation instrument used with the implant. A recess 74 islocated at the top of the hole and includes a keying notch 76 fororienting the targeting/installation instrument. See FIGS. 1 and 4. Therecess allows the screw head to install substantially flush with thesurface of the body to minimize the amount of bone removal required toinsert the stem into the humerus.

Body 18 includes a medially-facing inclined mounting surface 80 at theproximal end onto which head 12 is mounted. Head 12 is secured to body18 by coupling structure 82 which includes a fitting in the form of apedestal or dovetail 84 located on mounting surface 80. As shown inFIGS. 6 and 8, dovetail 84 is tapered from anterior to posterior toestablish a taperlock with the head, as will be described below. Becausethe dovetail is tapered, the body has a left or right orientationdepending on which shoulder is to be replaced. Thus, as shown in FIG. 2,the kit will preferably include two or more bodies. Additional bodies,over and above one left and one right, may be provided to accommodatedifferent stem diameters or head angles, etc.

Head 12, which is preferably formed as a unitary member, as opposed tobeing assembled from two or more components, includes a generallysemi-spherical articulation surface 90 which is adapted to engage theglenoid cavity in the shoulder. See FIGS. 10-13. Because the glenoidcavity does not define a close fitting socket, such as found in theacetabulum in the hip joint, the articulation surface only needs to besufficiently spherical to allow smooth articulation in the glenoidcavity.

As best shown in FIG. 14, articulation surface 90 is bounded by anarticular margin 92 which defines an articular plane 94 generally normalto a head axis 96. In the preferred embodiment, where the head issubstantially spherical, the head axis represents a central axis ofrotational symmetry for the articulation surface and a center ofcurvature 98 lies on the head axis. See FIG. 11. As shown by the dottedlines in FIG. 13, the various heads are preferably formed with the sameradius, but simply represent larger portions of a sphere. It is believedthat this best reflects the actual anatomical characteristics.

In the most commonly occurring fracture pattern, the anatomic headfractures generally through the articular margin and plane. Thearticular plane defines generally the distal extent of head 12. This isimportant when it is necessary to remove the head as part of a revisionprocedure, because the present invention allows the head to be removedfrom an anterior direction without dislocation of the joint and theassociated trauma. This is not the case with existing implant heads,which cannot be separated from the body for removal without firstdislocating the joint. It is desirable, although not required, that thehead not project substantially beyond the articular plane in the presentinvention so that it is possible to slide the head out of the joint inan anterior direction without disruption of the surrounding bone.Because the remainder of the humerus is distal to the articulationplane, the head may be slid out in that plane without disruption of thesurrounding bone as long as the head does not project substantiallybeyond the articular plane. Thus, the coupling structure is adapted toallow the head to be installed on and removed from the stem withoutdislocating the shoulder after the implant has been installed in theshoulder.

Head 12 includes a mounting surface or backside 100 disposed oppositethe articulation surface and separated from the articulation surface bythe articular margin. Backside 100 includes a portion of couplingstructure 82 in the form of a transverse track or undercut channel 102.Channel 102 is cut to match the cross-sectional shape and taper ofdovetail 84 and includes an open end 104 and an inner end 106. Acylindrical recess 108 extends from the perimeter of the head past theinner end of the channel and to a stop 110. A groove 112 is formed inrecess 108 near the edge of the head.

The channel is sized so that the head is guided onto the body and thedovetail taperlocks in the channel when the head is properly positioned.See FIG. 10. The taperlock connection is important because it rigidlysecures the components and prevents them from fretting against eachother and generating debris over time. The coupling structure of thepresent invention may also be described as a transversely acting taperlock, with a portion of the taper lock being disposed on the head and aportion disposed on the body. The taper lock of the present invention istransverse acting in that it does not rely on motion along the axis ofthe head to lock, contrary to existing designs. In fact, it can be seenthat, when the head is engaged on the stem, the coupling structuremechanically interlocks the head against motion transverse to thearticular plane. This is in contrast to existing designs, which simplyrely on a frictional interconnection in the direction transverse to thearticular plane.

A locking member in the form of a screw 114 is provided to draw the headfirmly onto the body to properly seat the taperlock. In particular,after head 12 is initially positioned on the body, as shown in FIG. 15,it is slid generally into position and screw 114 is installed into hole72 with the head of the screw fitting closely into cylindrical recess108. See FIG. 16. As the screw is driven in, the head of the screwengages stop 110 to pull head 12 firmly onto body 18. Screw 114 alsoserves as a backup interlock to insure that the head does not becomedislodged. It should be noted that the head of the screw will not seatcompletely against the body because some space must be left toaccommodate machining tolerances in the coupling structure so that thetaper lock may be drawn fight in all cases.

When it is necessary to remove the head, as in a revision, a tool 120with a flange 122 secured near the tip of the tool is utilized. See FIG.16. The tip of the tool is initially installed in the screw head from aslight angle away from the head and then the tool is rotated toward thehead to engage the flange with taper breaking surface in the form of agroove 112 formed in recess 108. As the screw is backed out, the flangepulls against the head to dislodge the taperlock. Thus, the head can beremoved with application of external force to the implant, as has beenrequired with prior designs. This reduces the chance that the entireimplant will be loosened when only the head needs to be removed.

Installation of the implant of the present invention is facilitated by atargeting/installation instrument, shown generally at 130 in FIG. 18.Instrument 130 includes a template member 132 to which are mounted amounting bar 134, a height adjusting mechanism 136 and a retroversionguide 138. Mounting bar 134 serves to join template member 132 toimplant 10. In particular, bar 134 is hollow and includes a tab 140 (notshown) at the free end. The bar receives a bolt 142 with a head 144 anda threaded end 146. To attach the instrument to the implant, the freeend of the bar is placed in recess 74 and aligned so that tab 140 fitsinto keying notch 76. This establishes the correct alignment between thetemplate and the implant. The threaded end of the bolt is then screwedinto hole 72 to secure the instrument to the implant. The bar includes aflat 148 to allow the bar to reach body 18 without engaging head 12. Inaddition, it should be noted that the screw which secures the head tothe body is not installed until after the instrument is removed.

Once the instrument is mounted to the implant, the stem is inserted intothe shaft of the humerus. In the typical fracture pattern, the head andgreater and lesser tubercles are separated from the remainder of thehumerus, leaving a pipe-like upper shaft. As a result, there is noremaining reference for the correct height of the implant head relativeto the top of the humeral shaft. It is important to position the head atthe correct height relative to the humeral shaft to avoid excess tensionon the deltoid muscle by having the head too high or deltoid lag wherethe head is too low and the deltoid must undergo some contraction priorto starting to move the arm.

The height adjusting mechanism allows the surgeon to temporarily set theheight of the head and then evaluate the deltoid tension. In particular,as shown in FIGS. 18 and 19, height adjusting mechanism 136 includes aguide bar 150 which is moveably mounted to a carriage 152, which isdriven up and down along a threaded rod 154. With the implant in ahumeral shaft 156, the guide bar is positioned to sit on top 158 of thehumeral shaft. The surgeon can then adjust the implant up or down byturning the threaded rod. The guide bar establishes a predeterminedheight, which can be maintained while retroversion is set and even ifthe implant is removed and reinserted, as when bone cement is used.

After establishing the correct height the surgeon can use theretroversion guide to set correct retroversion, as shown in FIG. 20. Theretroversion guide includes an L-shaped rod 160 with a lower sightingarm 162. Rod 160 is pivotally and slidably mounted to template 132 toallow the height and angular orientation of the sighting arm to beadjusted. A set screw 164 allows the position of the rod to be fixedonce it is in the desired orientation. In use, the sighting arm is setfor a predetermined retroversion angle relative to the head axis, forinstance 30-degrees. This can be accomplished before attachment to theimplant using a protractor jig (not shown). With the sighting arm set tothe correct orientation, the patient's forearm is flexed toapproximately 90-degrees to the humerus. The surgeon then rotates theimplant to align the sight arm with the axis of the forearm, therebyeasily and accurately establishing the desired retroversion.

Once the correct height and retroversion is established, a cannulateddrill guide 170 is inserted through guide holes 172 provided in thedistal end of the template member. See FIG. 18. Guide holes 172 areoriented to target locking holes 36 in the end of the stem. A drill 174is inserted though the drill guide to bore through the bone over thelocking holes. One or two screws are installed through the humerus andlocking holes to secure the implant in place.

As shown by the dotted lines in FIG. 19, it is possible to attach thegreater tubercle 159 to the implant prior to final securing of the head.This allows the surgeon to evaluate the tension in the rotator cuff andmake corrections, if necessary, by moving to a smaller or larger head.One other feature of the present invention is the provision of suturesupports 180, shown in FIG. 21, which serve to distribute the force ofthe suture over the bone. Particularly in trauma cases, the bone is verysoft and without supports 180, the sutures will sometimes pull throughthe bone. By utilizing the supports, the surgeon can obtain the desiredsuture tension without risk of the suture pulling through the surface ofthe bone.

It should be noted that the targeting/installation instrument isprovided in left and right versions, although it would also be possibleto make mounting bar 134 reversible or symmetric to accommodate left andright bodies. In addition, a longer template member would be used withthe longer shafts used to treat mid-shaft fractures.

While the invention has been disclosed in its preferred form, it is tobe understood that the specific embodiment thereof as disclosed andillustrated herein is not to be considered in a limiting sense asnumerous variations are possible and that no single feature, function orproperty of the preferred embodiment is essential. The invention is tobe defined only by the scope of the issued claims.

I claim:
 1. A shoulder prosthesis comprising:a head having asemi-spherical articulation surface shaped to articulate with a glenoidcavity and bounded by an articular margin disposed in an articular planegenerally normal to a head axis, the head further including a backsidedisposed opposite the articulation surface and separated from thearticulation surface by the articular margin; a stem portion including aproximal end and a distal shaft for insertion into a medullary canalalong a shaft axis; and a coupling structure in the form of atransversely acting taper lock, with a first portion of the taper lockbeing mounted on the head and a second portion of the taper lock beingmounted on the proximal end of the stem, the taper lock being adapted totighten under motion generally transverse to the head axis.
 2. Theprosthesis of claim 1, wherein the stem portion further includes aseparable body portion, where the body portion forms the proximal end ofthe stem.
 3. The prosthesis of claim 1, wherein the stem portion iscannulated.
 4. The prosthesis of claim 1, wherein the transverselyacting taper lock includes a transversely oriented undercut channel anda mating pedestal that slideably engages the channel.
 5. The prosthesisof claim 4, wherein the channel and pedestal are dovetail-shaped.
 6. Theprosthesis of claim 4, further including a selectively operable lockingmember associated with the coupling structure and adapted to tighten thetaper lock upon operation.
 7. The prosthesis of claim 4, wherein thepedestal is positioned on the proximal end of the stem and the channelis formed in the backside of the head.
 8. The prosthesis of claim 1,wherein the stem has a central axis and the proximal end of the stemincludes at least one transverse bore formed medial of the central axisfor receipt of a suture.
 9. The prosthesis of claim 1, furthercomprising a locking screw associated with the coupling structure toprevent the taper lock from disengaging when the screw is installed, thehead having a taper breaking surface which the screw can selectivelyoperate against upon removal of the screw to disengage the taper lock.10. The prosthesis of claim 1, wherein the coupling structure is adaptedto allow the head to be installed on and removed from the stem withoutdislocating the shoulder after the implant has been installed in theshoulder.
 11. A shoulder prosthesis comprising:a unitary head having asemi-spherical articulation surface bounded by an articular margindisposed in an articular plane generally normal to a head axis, the headfurther including a backside disposed opposite the articulation surfaceand separated from the articulation surface by the articular margin, astem portion including a proximal end and a distal shaft for insertioninto a medullary canal along a shaft axis; and a coupling structure withan undercut track portion and a corresponding pedestal portion adaptedto engage the undercut track for constrained motion along a couplingaxis to a locked position, where one of the portions is incorporated inthe head and one of the portions is incorporated in the stem with thecoupling axis being oriented generally parallel to the articular plane.12. The prosthesis of claim 11, wherein the coupling axis is furtheroriented transverse to the shaft axis.
 13. The prosthesis of claim 11,wherein the stem portion further includes a separable body portion,where the body portion forms the proximal end of the stem portion. 14.The prosthesis of claim 11, wherein the track and pedestal portions aremutually tapered to establish a taper lock in the locked position.
 15. Ashoulder prosthesis comprising:a head having an articulation surfaceconfigured to articulate with the glenoid cavity and a backside opposedto the articulation surface; a stem portion including a proximal end anda distal shaft for insertion into a medullary canal along a shaft axis;and a coupling structure disposed partially on the backside of the headand partially on the proximal end of the stem portion and adapted toreleasably couple the head to the proximal end of the stem portion, thecoupling structure being configured to allow the head to be installed onand removed from the stem portion without shifting the headsubstantially away from the stem portion in the direction of the glenoidcavity.
 16. The prosthesis of claim 15, wherein the coupling structureallows the head to be installed and removed from an anterior direction.17. The prosthesis of claim 15, wherein the stem portion furtherincludes a separable body portion, where the body portion forms theproximal end of the stem portion.
 18. The prosthesis of claim 17,wherein the body includes a socket to receive a proximal tapered end ofthe distal shaft.
 19. The prosthesis of claim 18, wherein the bodyincludes an alignment member to establish a predetermined rotationalorientation between the body and the shaft.
 20. A shoulder prosthesiscomprising:a head having a semi-spherical articulation surface boundedby an articular margin disposed in an articular plane generally normalto a head axis, the head further including a backside disposed oppositethe articulation surface and separated from the articulation surface bythe articular margin; a stem portion including a proximal end and adistal shaft for insertion into a medullary canal along a shaft axis;and a coupling structure disposed partially on the backside of the headand partially on the proximal end of the stem for guiding the head in aconstrained motion along a coupling axis to a locked position, with thecoupling axis being oriented generally transverse to both the head axisand the shaft axis.
 21. The prosthesis of claim 20, wherein the stemportion further includes a separable body portion, where the bodyportion forms the proximal end of the stem portion.
 22. The prosthesisof claim 21, wherein the portion of the coupling structure on the bodyis asymmetric relative to an anterior/posterior plane, whereby the bodyis particularly adapted for one side of a patient's body.
 23. A shoulderprosthesis comprising:a head having a semi-spherical articulationsurface bounded by an articular margin disposed in an articular planegenerally normal to a head axis, the head further including a backsidedisposed opposite the articulation surface and separated from thearticulation surface by the articular margin; a stem portion including aproximal end and a distal shaft for insertion into a medullary canalalong a shaft axis; and a coupling structure disposed partially on thebackside of the head and partially on the proximal end of the stem forreleasably coupling the head to the stem; and a locking elementconfigured to engage and interlock the head and the stem to prevent themfrom decoupling, the locking element being installable and removeablefrom an anterior direction without dislocating the shoulder.
 24. Theprosthesis of claim 23, wherein the head includes a recess to receive atleast a portion of the locking element, the recess being orientedgenerally parallel to the articular plane to allow the locking elementto be accessed after installation of the implant in the shoulder withoutsignificant disruption of bone around the prosthesis.
 25. The prosthesisof claim 23, wherein, when the head is engaged on the stem, the couplingstructure mechanically interlocks the head against motion transverse tothe articular plane.